Low frequency loudspeaker



July 9, 1963 s. J. zuERKER Low FREQUENCY LOUDSPEAKER 2 Sheets-Sheet 1 Filed July 28, 1959 July 9, 1963 s. J. ZUERKER 3,097,268

Low FREQUENCY LOUDSPEAKER United States Patent Oice 3,097,268 Patented July 9, 1963 3,097,263 LW FREQUENCY LOUDSPEAKER Siegfried J. Zuerker, Utica, NX., assignor to General Electric Company, a corporation of New York Filed .luiy 28, 1959, Ser. No. 830,031 2 Claims. (Cl. 179-1155) This invention relates to loudspeakers for producing sound from electrical signals, and more particularly to loudspeakers of the moving-coil type intended for reproducing the lower-frequency audio signals.

In loudspeakers of the moving-coil type, in which the coil is attached to a cone or diaphragm, the lowest audio frequency that the loudspeaker is capable of reproducing is determined by the resonant frequency of the moving system which consists of the combination of the cone, voice coil, and suspension system. As a practical matter, no useful output is produced at frequencies below the resonant frequency of the loudspeaker. The resonant frequency of a low-frequency type of loudspeaker generally lies somewhere between 30 cycles per second and 100 cycles per second. The resonant frequency of a loudspeaker depends primarily on the mass of the cone, and the compliance (i.e. the stiffness) of the suspension structure which supports the cone. Loudspeakers have been designed with massive cones, and with relatively loose cone suspensions, in order to lower the value of the resonant frequency so that the loudspeakers will be capable of reproducing lower frequencies of sound. However, there are practical limitations to these expedients, and, therefore, there has been an urgent desire for a new technique that will reduce the resonant frequency without the necessity for changing the cone mass and/or the suspension compliance from their optimum values as determined by other considerations.

An object of the present invention is to provide ya new and improved loudspeaker construction for achieving a low value of resonant frequency, thereby providing for improved reproduction of low-frequency sounds.

A further object is to provide a loudspeaker having improved low-frequency response, and at the same time having its cone mass and suspension compliance chosen for optimum performance based on considerations other than the resonant frequency. Still other objects of the invention will become apparent from the description and claims which follow, and from the accompanying drawing.

In accordance with the invention, the resonant frequency of a loudspeaker is lowered by providing the loudspeaker with a voice coil having a greater mass than has heretofore been contemplated. The mass of the voice coil is determined primarily by the amount of wire used in winding the voice coil, and is determined to a lesser extent by the weight of the cylindrical form on which the winding is placed. The term voice coil is to be understood to include both the winding and the form on which the winding is positioned, this form customarily being attached to the apex of a cone-shaped diaphragm yand being positioned in a iixcd magnetic lield, the ends of the voice coil winding being connected to the output terminals of an audio amplier.

-It has been known that the mass of the voice coil affects the eciency of a loudspeaker; however, it has not heretofore been realized that the mass of the voice coil, if made great enough, will have a very beneficial effect on the resonant frequency of the loudspeaker.

In accordance with the prior art, voice coils are designed to have only a sufficient mass to achieve an adequate efficiency of operation. As an example of the prior art in this respect, Patent No. 2,445,276 to Frank Massa, issued July 13, 1948, shows, in FIG. 4 thereof, that if the mass of the voice coil is increased to a certain limit, the etliciency of the loudspeaker will be increased, and beyond the value of mass for peak eciency, the efficiency will drop if the mass were to be increased further. According to the teachings of the Massa patent, a voice coil should have a mass at or slightly below the value that will cause peak efficiency to occur, but this mass should not be greater than the value at which peak efficiency is produced. Contrary to these prior-art teachings, the loudspeaker construction of the present invention employs a voice coil having a mass which is greater than that which will produce peak efficiency, and it is discovered :that by utilizing such a voice coil, the resonant frequency of the loudspeaker is caused to be lowered a substantial amount, thereby achieving the beneficial result of extending the low-frequency range of the loudspeaker.

In the drawing:

FIG. 1 is a front view of a loudspeaker in accordance with the present invention;

FIG. 2 is a side view, shown partially in cross-section, of FIG. 1;

FIG. 3 is a sectional view of the voice coil region of the loudspeaker of FIGS. 1 and 2; and

FIG. 4 is a graph showing the principles of operation of the invention.

Now referring to FIGS. 1, 2 and 3, the loudspeaker comprises a frame 10' made of metal or other suitable material and having a rim 11 at one end thereof and a flange 12 at the other end thereof. A magnetic structure 13 is attached to the flange 12 and comprises a front plate 14 to which the flange 12 is spot welded or otherwise suitably attached. A hollow cylindrical permanent magnet 15 is positioned against the rear surface of the front plate 14, and a rear plate 16 is positioned against the rear end of the magnet 15. Bolts 17 may be used for clamping the front and rear plates 14, 16 against the ends of the cylindrical magnet 15. A core 18 is attached to the rear plate 16, such as by being forced-fitted into an opening in the rear plate 16. This core 18 projects towards the front plate 14, and the forward end thereof has a circular crosssectional shape and constitutes a center pole 19 which is centered within a circular opening in the front plate 14 thereby providing a circular air gap 21. The region of the front plate 14 which surrounds the gap 21, constitutes an outer pole 22. The front and rear plates 14, 16 and the core 18, are made from iron or other suitable material through which magnetic flux from the magnet 17 can flow, thereby providing an intensive magnetic ux in a radial pattern in the gap 21 between the center pole 19 and the outer pole 22. The core 18 preferably is provided with a groove 23 adjacent to the center pole 19, to aid in concentratina the flux in the air .can 21.

A cone 26, which may have a generally conical shape and which may be made from pressed paper or other suitable material, is attached to the rim 11 -by means of an outer suspension 27 which may com-prise a series of concentric folds lof the outer part of the cone 26. A voice coil form 28, of cylindrical shape and which may be made of thin paper, or a non-:magnetic metal such as aluminum, is attached to the co-ne 26 at the apex thereof, and extends into the air gap 21 between the center pole 19 and the outer pole 22. An inner suspension 29 is provided between the frame 10 and the forward end of the voice coil form 28, and may comprise impregnated cloth material shaped in the form of a series of concentric folds, as shown. A dust cover 31, which m-ay be made from paper or aluminum or other suitable material, -is sealed to` the cone 26 near the apex thereof so as to cover over the Voice coil form 28.

A voice coil wind-ing 33 is positioned on the voice coil form 2.8, and comprises `a plurality of turns of wire Wound around, or otherwise attached to, the outer surface of the form 28, and so located with respect thereto that some or all of the winding 33 is positioned in the magnetic ux in the air gap 21 when the cone 26 is in its neutral position. The voice coil 33 is preferably wound with copper wire, although wire of aluminum or other electrically conductive material may be used. As shown, the voice coil 33 may constitute a plurality of layers, and its length may be greater than the length of the gap 21, as shown. The ends of the voice coil winding 33 are connected, via lead .wires 34, to suitable electrical terminals 35, to which connections can be made from an audio amplifier.

When electrical signals are applied to the voice coil 33, from an audio amplier or other suitable source, the current thereby created in the winding 33 will react with the magnetic flux in the gap 21 in a well-known manner so as to cause the cone 26 to vibrate in accordance with the electrical signals, thereby Igenerating acoustical sound waves in accordance with the electrical signals in the winding 33. The stiffness of the outer suspension 27 and the inner suspension 29 are critical for optimum performance, as is well known. If these suspensions have too much stiffness, i.e., low compliance, the cone 26 will not 'respond readily to low-frequency electrical signals in the winding 33. On the other hand, if the suspensions have too great a compliance, i.e., are not suiciently stiff, the voice coil winding 33 will not maintain a proper axial position in the gap 21, and will rub against the pole pieces during operation thereof.

As has been mentioned above, in accordance with the prior art, the combined value ofthe mass of the winding 33 and the voice coil form 28 would have a value equal to or less than the value which achieves maximum eiciency for the loudspeaker. In accordance with the present invention, however, the combined mass of the winding 33 and form 28 has a value greater than that at which maximum eiciency is achieved. This increased mass preferably is accomplished by using a larger diameter of wire for the windinlg 33 than would normally be used, and/ or by using more turns of wire for the voice coil winding 33, the desideratum being to put more material, and hence more m-ass, into the winding 33 than has heretofore been considered desirable.

FIG. 4 illustrates how the new type of loudspeaker construction, in accordance with the present invention, functions to reduce the value of the resonant frequency. FIG. 4 shows, by way of example, typical curves for a loudspeaker having a cone diameter of l inches. Curve 41 shows how the efficiency `of the loudspeaker, as a percentage, varies with the mass of the voice coil. This is the same curve as shown in FIG. 4 of `the aforementioned Massa patent for a loudspeaker having a copper voice coil winding and a cone diameter of inches. The etiiciency curve has a peak 45, at which the loudspeaker achieves maximum conversion of electrical energy into acoustical sound energy. According to the said Massa patent, and in accordance with the prior art, the voice coil should have a mass lying in the range between and i.e., between 4 Igrams and 20 grams for a Iloudspeaker having a l0 `inch cone, as indicated by the prior art horizontal bracket in FIG. 4. In accordance with the present invention, the voice coil is given a mass greater than that for maximum efficiency, i.e., for a l0 inch loudspeaker cone as shown in FIG. 4 this mass would be greater than a value of about 25 grams, as indicated by the invention horizontal bracket.

The curve 42 `in FIG. 4 shows how the loudspeaker resonant frequency varies with the mass of the voice coil.

This curve shows that, when the voice coil has a mass in the range of lthe prior art, the cone resonant frequency is substantially constant. However, in accordance with the present invention, it is found that the loudspeaker resonant frequency becomes substantially lower as the voice Icoil mass is increased above the mass required for maximum eiiiciency and, as shown in FIG. 4, if the voice coil is constructed to have a mass greater than that required for a maximum eciency, the result will be a lower loudspeaker resonant frequency, and hence, with this construction, the loudspeaker will be capable of responding to lower frequencies, thereby providing a more faithful and higher -delity reproduction of the program material that is fed to the loudspeaker.

Although .the invention contemplates a loudspeaker construction in which the mass of the voice coil is any desired value greater than the mass at which peak efficiency is achieved, the most practical range of voice coil mass to `be used in practicing the invention, lies between the value of mass corresponding to the peak 45 of the elficiency curve 4l and the value of mass corresponding to the lower knee 43 of the cone resonant frequency curve 42. The upper knee 44 of the eiciency curve approximately coincides with the peak 45 of the efficiency curve 4]., and there is relatively little change in resonant frequency to the left of this region, whereas the resonant frequency becomes substantially lower to the right of this region. Although the loudspeaker of the invention operates at less than peak efliciency, this is not a serious consequence in View rof the important accomplishment of a greatly lowered resonant frequency.

-It should-be understood that the curves shown in FIG. 4 and used in explaining the functioning of the invention, are representative curves for a particular loudspeaker having a 10 inch cone, and the exact magnitudes and shapes of these curves may vary in accordance with changes in cone size, cone mass, the suspension compliance, and the strength of the magnetic ux in the gap 21, and also upon the type of enclosure in which the loudspeaker is mounted.

While a preferred embodiment of the invention has been shown and described, other embodiments and modications thereof will be apparent to those skilled in the art, and will fall within the scope of the invention as dened in the following claims.

What -I claim as new and desire to secure by Letters Patent of the United States is:

-1. A loudspeaker comprising a cone of generally conical shape, a voice coil of generally cylindrical shape attached to said cone at the apex thereof, said voice coil comprising a winding arranged coaxially with the axis of said cone, and means for supplying a magnetic field radially through said winding, said loudspeaker comprising means for producing a characteristic curve of eiciency versus voice coil mass having a peak value for a given mass of said voice coil and a characteristic curve of resonant frequency versus voice coil mass having relatively little change in frequency at voice coil masses less than the mass that produces the peak efliciency and thereafter descending to lower resonant frequencies as the mass of the voice coil is increased and further having a knee at a value of Voice coil mass greater than that required for peak eciency, said voice coil comprising sufcient material so as to have a mass lying between that at which said peak eiciency occurs and that which corresponds to said knee of the loudspeaker resonant lfrequency curve.

2. A loudspeaker comprising a cone of generally conical shape, a voice coil of generally cylindrical shape attached -to said cone at the apex thereof, said voice coil comprising a winding arranged coaxially with the axis of said cone, and means for supplying a magnetic eld radially through said winding, said loudspeaker comprising means for producing a characteristic curve of efliciency versus voice coil mass having a peak value for a given mass of said voice coil and a characteristic curve of resonant frequency versus voice coil mass having relatively little change in frequency at voice coil masses less than the mass at which peak efficiency occurs and having an upper knee at a voice coil mass in the region of said peak eflciency and thereafter descending to lower cone resonant frequencies as the mass of the voice coil is increased and further having a lower knee at a value of voice coil mass greater than that required for peak ecency, said voice coil comprising sui'lcient material so as to have a mass Igreater than that which corresponds to said peak value of eciency and lying between said lower knee and said upper knee of the loudspeaker resonant `frequency curve.

References Cited in the file of this patent UNITED STATES PATENTS 2,007,484 Tolerton July 9, 1935 2,445,276 Massa July 13, 1948 2,769,942 Hassan Nov. 6, 1956 OTHER REFERENCES Hi-Fi Loudspeakers and Enclosure, by A. B. Cohen, A Rider Publication, No. 176, pp. 47, 49, 82, 83, 85, 93,

10 and 95.

Applied Acoustics, by Olson and Massa, 2nd ed., pp. 23S-240. 

1. A LOUDSPEAKER COMPRISING A CONE OF GENERALLY CONICAL SHAPE, A VOICE COIL OF GENERALLY CYLINDRICAL SHAPE ATTACHED TO SAID CONE AT THE APEX THEREOF, SAID VOICE COIL COMPRISING A WINDING ARRANGED COAXIALLY WITH THE AXIS OF SAID CONE, AND MEANS FOR SUPPLYING A MAGNETIC FIELD RADIALLY THROUGH SAID WINDING, SAID LOUDSPEAKER COMPRISING MEANS FOR PRODUCING A CHARACTERISTIC CURVE OF EFFICIENCY VERSUS VOICE COIL MASS HAVING A PEAK VALUE FOR A GIVEN MASS OF SAID VOICE COIL AND A CHARACTERISTIC CURVE OF RESONANT FREQUENCY VERSUS VOICE COIL MASS HAVING RELATIVELY LITTLE CHANGE IN FREQUENCY AT VOICE COIL MASSES LESS THAN THE MASS THAT PRODUCES THE PEAK EFFICIENCY AND THEREAFTER DESCENDING TO LOWER RESONANT FREQUENCIES AS THE MASS OF THE VOICE IS INCREASED AND FURTHER HAVING A KNEE AT A VALUE OF VOICE COIL MASS GREATER THAN THAT REQUIRED FOR PEAK EFFICIENCY, SAID VOICE COIL COMPRISING SUFFICIENT MATERIAL SO AS TO HAVE A MASS LYING BETWEEN THAT AT WHICH SAID PEAK EFFICIENCY OCCURS AND THAT WHICH CORRESPONDS TO SAID KNEE OF THE LOUDSPEAKER RESONANT FREQUENCY CURVE. 